"We are three states, two countries, and one reason. And as I've been saying, there's no other region like El Paso/Juarez," Margo said.

A Justice Department official said the federal government is treating the shooting as a domestic terrorism case. U.S Attorney John Bash also said that the federal government would investigate the attack in order to bring forward federal hate crime and federal firearms charges.

Local officials said Monday that 15 victims are still being treated at the hospital, including two who are still in critical condition.

Of the 22 fatalities, seven are Mexican nationals, one is from Germany, 13 are American citizens and one victim's nationality has yet to be determined.

A racist, anti-immigrant screed posted online shortly beforehand was attributed to the man arrested in the attack on the 680,000-resident border city, according to El Paso Police Chief Greg Allen. That man is 21-year-old Patrick Crusius of Allen, Texas, who has been booked into the El Paso County Jail on capital murder charges, and authorities are seeking the death penalty.

Crusius was arrested without police firing any shots outside the Walmart near the Cielo Vista Mall, about five miles from the main border checkpoint with Ciudad Juárez, Mexico. He was on foot when he was taken into custody.

The FBI has activated their domestic terrorism fusion cell and carried out three search warrants related to the shooting.

Mugshot of Patrick Crusius, 21, who is charged with capital murder for the El Paso mass shooting on August 3, 201.

Police said they did not know where the weapon was purchased. Allen acknowledged that it is legal under Texas law to carry a long gun openly in a public place.

"Of course, normal individuals seeing that type of weapon might be alarmed," but before he began firing, the suspect was technically "within the realm of the law," El Paso Police Chief Greg Allen said.

"The scene was a horrific one," said Chief Allen, adding that many of the 26 people who were hurt had life-threatening injuries.

"It's chaos right now," said Austin Johnson, an Army medic at nearby Fort Bliss, who volunteered to help at the shopping center and later at a school serving as a reunification center.

The shooting occurred at the Walmart and its parking lot, and no related incidents happened elsewhere in the Cielo Vista Mall, according to El Paso Police Department Sgt. Robert Gomez.

All of the bodies had been removed from the crime scene by Sunday evening, and FBI spokesperson Jeanette Harper said crime scene forensic review is underway.

Residents quickly volunteered to give blood to the injured after the El Paso shooting, and police and military members were helping people look for missing loved ones.

In a press conference Sunday evening, Gomez said the blood donations have been 'overwhelming' and asked future donors to make appointments.

Adriana Quezada, 39, said she was in the women's clothing section of Walmart with her two children when she heard gunfire.

"But I thought they were hits, like roof construction," she said of the shots.

Her 19-year-old daughter and 16-year-old son threw themselves to the ground, then ran out of the store through an emergency exit. They were not hurt, Quezada said.

She said she saw four men, dressed in black, moving together firing guns indiscriminately. Police later said they believed the suspect, who was armed with a rifle, was the only shooter.

Ryan Mielke, a spokesman for University Medical Center of El Paso, said 13 of the injured were brought to the hospital with injuries, including one who died. Two of the injured were children who were being transferred to El Paso Children's Hospital, he said. He wouldn't provide additional details on the victims.

Eleven other victims were being treated at Del Sol Medical Center, hospital spokesman Victor Guerrero said. Those victims' ages ranged from 35 to 82, he said.

Relatives said a 25-year-old woman who was shot while holding her 2-month-old son was among those killed, while Mexican officials said three Mexican nationals were among the dead and six more were wounded.

President Donald Trump ordered American flags in the U.S. and around the world lowered to half staff until Aug. 8, "as a mark of solemn respect" for both the victims of the shooting in El Paso as well as victims of a shooting in Dayton, Ohio, that occurred less than 24 hours later.

"There are no reasons or excuses that will ever justify killing innocent people," also Trump wrote.

At a candidate forum Saturday in Las Vegas, presidential candidate and former Texas congressman Beto O'Rourke, who is from El Paso, appeared shaken after news of the shooting in his hometown was reported.

O'Rourke said he heard early reports that the shooter might have had a military-style weapon, saying we need to "keep that (expletive) on the battlefield. Do not bring it into our communities."

Texas Gov. Greg Abbott described the shooting as one of the deadliest days in the state's history.

"Texas grieves for the people of El Paso today," he said during a press conference, during which authorities confirmed the number of people killed and wounded. "On a day that would have been a normal day for someone to leisurely go shopping... Lives were taken who should still be with us today.”

When asked about the manifesto, Abbott said "This is disgusting, intolerable. It's not Texan. And we are going to aggressively prosecute it as capital murder and as a hate crime, which is exactly what it appears to be, without having seen all of the evidence yet."

El Paso Mayor Dee Margo said he knew the shooter was not from his town.

"It's not what we're about," he said at the news conference with Abbott. El Paso is nearly a 10-hour drive from Allen.

In the hours after the shooting, authorities blocked streets near a home in Allen associated with the suspect. Officers appeared to speak briefly with a woman who answered the door of the gray stone house and later entered the residence.

El Paso County is more than 80% Latino, according to the latest census data, and the city, where the mayor said tens of thousands of Mexicans legally cross the border each day to work and shop, has become a focal point of the immigration debate. Trump visited in February to argue that walling off the southern border would make the U.S. safer, while city residents and O'Rourke led thousands on a protest march past the barrier of barbed wire-topped fencing and towering metal slats.

O'Rourke stressed that border walls haven't made his hometown safer. The city's murder rate was less than half the national average in 2005, the year before the start of its border fence. Before the wall project started, El Paso had been rated one of the three safest major U.S. cities going back to 1997.

Heidi Beirich, director of the Southern Poverty Law Center's Intelligence Project, said the El Paso shooting suspect wasn't on her group's radar before the shooting.

"We had nothing in our files on him," Beirich wrote in an email.

People attend a vigil for victims of the shooting Saturday, Aug. 3, 2019, in El Paso, Texas. A young gunman opened fire in an El Paso, Texas, shopping area during the busy back-to-school season, leaving multiple people dead and more than two dozen injured. (AP Photo/John Locher)

A donation page has been created to support the victims. The El Paso Victims Relief Fund is the only official such page, police said. It was established by the Paso del Norte Community Foundation, which supports nonprofit organizations and charitable causes in the region.

The shooting came less than a week after a 19-year-old gunman killed three people and injured 13 others at the popular Gilroy Garlic Festival in California before dying of a self-inflicted gunshot wound.